Uranium-series age determinations by mass spectrometric methods were done for travertines and associated carbonate veins related to clastic deposits of the last glaciation (Pinedale) in the northern Yellowstone area. Dramatic variations in the hydrologic head are inferred from variations in the elevation of travertine deposition with time and are consistent with the expected hydrologic effects of glaciation. We determine the following chronology of the Pinedale Glaciation, with the key assumption that travertine deposits (and associated carbonate veins) perched high above present thermal springs were deposited when glaciers filled the valley below these perched deposits: (1) the early Pinedale outlet glacier advanced well downvalley between 47,000 and 34,000 yr B.P.; (2) the outlet glacier receded to an interstadial position between 34,000 and 30,000 yr B.P.; (3) an extensive Pinedale ice advance occurred between 30,000 and 22,500 yr B.P.; (4) a major recession occurred between 22,500 and 19,500 yr B.P.; (5) a minor readvance (Deckard Flats) culminated after 19,500 yr B.P.; and (6) recession from the Deckard Flats position was completed before 15,500 yr B.P. This chronology is consistent with the general trend of climatic changes in the northern hemisphere as revealed by recent high-resolution ice-core records from the Greenland ice sheet.